Origin of leucosomes in a Kyanite-Zone migmatite of the Higher Himalayan Crystallines, Bhagirathi Valley (NW Himalaya)

Abstract

This study encompasses field observations, petrography, whole-rock geochemistry, zircon and monazite geochronology, and oxygen isotope analysis to delineate the conditions and timing of partial melting in the Higher Himalaya Crystallines (HHC) along Bhagirathi Valley. Two distinct leucocratic layers (L-1 and L-2) were identified and studied to elucidate their formation mechanisms. Field evidence and petrographic studies indicate that the L-1 layer represents Tourmaline-bearing leucogranite (TBL) veins that infiltrated along extensional shear bands, while L-2 represents in-situ leucosomes generated during the partial melting of metasedimentary rocks. The peak metamorphic conditions were estimated at ∼ 720–740 °C and 1.0 GPa, which suggests that L-2 melts originated by fluid-assisted melting, rather than by closed system dehydration melting. Phase equilibria modelling suggests that protolith bulk composition, particularly the Na/K and Al/Si ratios, plays a critical role in melt chemistry, with elevated Na/K ratios in the host-rock promoting trondhjemite melts while lower Na/K ratios produce granitic melts. Zircon and monazite geochronology document two metamorphic episodes: an event at ∼ 35 Ma, which possibly represents melt generation at greater depth, and a subsequent episode at ∼ 21 Ma representing emplacement due to extensional tectonics. In-situ oxygen isotope analysis of zircon overgrowth suggests that intrusion of L-1 was accompanied by assimilation of surrounding metapsammite, recorded by a progressive increase in δ¹⁸O values from 7.5 ‰ at ∼ 25 Ma to 8.7 ‰ at |∼ 23 Ma. The high δ¹⁸O value (8.7 ‰) recorded by L-2 bearing migmatite is consistent with an origin by partial melting of a metapsammite protolith.